Field of the Invention
The field of the present invention is that of items of variable-geometry equipment and the control of their positioning, and more particularly that of the items of variable-geometry equipment mounted on turbomachines.
Description of the Related Art
Several items of equipment on a turbomachine can be mounted so as to be able to be moved in rotation or in translation, whether it be for example of the variable-setting stator blades which are oriented more or less in the direction of the air stream of a high-pressure compressor, of the discharge valves which are opened or closed so as to increase, if necessary, the surge margin of a compressor or else of the movable blades of a compressor or of a variable-pitch propeller.
These items of variable-geometry equipment are moved by kinematic systems actuated by jacks or any other actuator, as indicated in patent application EP 1724472 by the applicant, and their position is defined by a law establishing a relationship between the stroke of the jack and the position desired for the item of equipment in question. Moreover, a device for controlling the position of the item of variable-geometry equipment is known, as described in application EP 1988258, wherein the position of the item of equipment is detected by an end-of-stroke sensor.
The position is very often driven by a closed-loop control which loops on the desired position. The position of the rod of the jack is known by virtue of a sensor of the LVDT (for linear variable displacement transducer) kind which follows the movements of the jack rod and transmits them to the device for controlling the position of the kinematic system. These sensors are usually sensors of the absolute positioning type, that is to say that they indicate the position of the jack rod relative to a fixed reference, associated with the body of the jack or with the item of equipment itself.
This closed-loop control usually loops on the position of the jack rod and not on that of the item of equipment in question. The stroke of the jack may be perfectly controlled but correspond to an incorrect position of the item of equipment if clearances appear, for example by wear throughout the life of the machine, or else if major damage such as a breakage occurs in the kinematic system placed downstream of the jack.
As an example, wear is frequently encountered in the retaining bushes of the variable-setting stator blades (or variable stator vanes—VSV) and, consequently to see a considerable clearance in their positioning and a lack of precision in the control of this item of equipment.
The lack of precision in the positioning of the current systems leads the designers of turbomachines to provide margins in the dimensioning of the law for controlling items of variable-geometry equipment. Specifically they must take account of the difference that is likely to appear during the life of an item of equipment between its real position and that which the control system wishes it to take. In the particular case of an item of compressor equipment, for which protection should be taken in use against the surging phenomenon, the corresponding margins resulting in an adaptation of the compressor that is not optimal, the operating point being chosen further from the surging line than it should be, in order to take account of future wear and of the tolerance deviations associated with the lack of precision in the setting of the blades that results therefrom.
Moreover, during installation, the operator must ensure a precise positioning of the item of equipment in order to avoid further increasing the lack of precision in the positioning in operation. Such an operation is time-consuming and may cause possible handling errors.